Low-pressure mercurcy lamps are used as light sources in many optical systems. Some applications only use light coming from a small emission region of a lamp and only within a narrow range of angles about a viewing axis. For example, a microspectroreflectometer may typically use light within a 0.8.degree. half-angle cone radiating from a 100 .mu.m square region on the lamp. In such cases, there is little concern regarding the total optical power or radiant flux emitted by the lamp, but only about the power emitted into this limited acceptance region, i.e. the "brightness" of the lamp. it may also be important to maximize the brightness of a particular spectral line or of a narrow range of wavelengths of emitted light, rather than the brightness over the entire spectrum of light emitted by the lamp. In the case of low-pressure mercury lamps used in microspectroreflectometers and similar applications, the 253.7 nm mercury line is usually the wavelength of particular concern.
In U.S. Pat. No. 2,763,806, Anderson describes an end-viewed discharge tube surrounded by a separate fused envelope. The mercury discharge passes from one electrode in the discharge tube through the tube and returns through the annular region between the outside of the discharge tube and the inside of the envelope to another electrode in the annular region. Electrodes are hermetically sealed through the non-viewing end of the envelope. Compared to a conventional Penray lamp, made of drawn double-bore capillary tubes, in which the discharge is viewed from the side, an end-viewed lamp such as Anderson's with a 1 mm capillary bore discharge tube is about four times brighter, each lamp operating at a current that optimizes its brightness.
It is known from published literature that for the large-bore discharge tubes used in common fluorescent lamps, the mercury discharge is brightest when operating in a temperature range of 10.degree.-50.degree. C. In U.S. Pat. No. 4,325,006, Morton describes a low-pressure Xenon flash lamp which is viewed from the side. The lamp has an annular discharge volume between inner and outer tubes so that coolant may flow through the center as well as outside of the outer tube.
It is an object of the present invention to produce an envelope structure for a discharge lamp which gives a brighter light output.